WASHINGTON (AFP) - The US Supreme Court blocked President Barack Obama's bid to shield millions of migrants from deportation on Thursday and thrust the charged issue to the front line of the battle to succeed him.
Justices in the under-strength court were split 4-4 over Obama's bid to change immigration policy by executive action, thus leaving lower court rulings blocking the effort in place.
Obama dubbed the ruling "heartbreaking for the millions of immigrants who made their lives here" but warned his opponents they will not be able to thwart their dreams for much longer.
"In November, Americans have to make a decision about what we care about and who we are," he declared, in a nod to the White House race between Democrat Hillary Clinton and Republican Donald Trump.
The court normally has nine members, but Justice Antonin Scalia died in February and the Senate has refused to vote on Obama's nominee, appeals court judge Merrick Garland, to replace him.
The deadlock leaves Obama's immigration policy in limbo, like the fate of four million undocumented immigrants who stood to be given US work permits under the politically controversial plan.
"I promise you this, though, sooner or later immigration reform will get done. Congress is not going to be able to ignore America forever," Obama said, throwing down the gauntlet to Republicans.
"The fact that the Supreme Court wasn't able to issue a decision today doesn't just set the system back further - it takes us further from the country that we aspire to be," he warned.
Clinton, Obama's former secretary of state and the presumptive Democratic presidential nominee, was also quick to pivot from the defeat to the electoral battle ahead.
"Today's heartbreaking SCOTUS immigration ruling could tear apart five million families facing deportation. We must do better," she tweeted, even as Republicans celebrated the ruling.
Trump, a billionaire property mogul, was meanwhile promoting a golf course in Scotland and did not immediately react, but he has made his hardline views on immigration clear.
In some of the toughest anti-immigrant rhetoric ever used by a major party candidate, he has vowed to build a wall along the Mexican border to keep out "rapists" and drug dealers.
He has also promised to suspend all immigration by Muslims and by people "from areas of the world where there is a proven history of terrorism against the United States, Europe or our allies."
Clinton, by contrast, has vowed that as president, she would "create a pathway to citizenship, keep families together, and enable millions of workers to come out of the shadows."
Congressional Republicans have not all echoed Trump's stance, but welcomed the Supreme Court stalemate as a victory for efforts to stop Obama exceeding his executive authority to bypass Congress.
"Obama's illegal action on immigration has been blocked! A huge win for our Constitution and for our rule of law," tweeted Congressman Tom Marino, echoing the views of many of his colleagues.
Frustrated by Congress's repeated failure to pass immigration reform, in November 2014, Obama issued a decree to allow migrants whose children are legally resident to apply for permits.
This would have shielded the families from deportation while the politically-charged issue of their status is determined, but the governors of 26 Republican-led states challenged the order.
Federal courts in Texas and Louisiana put the measure on hold and the case passed to the Supreme Court, which on Thursday remained split along progressive-conservative lines.